AN IMPROVED PROCESS FOR SINTERING ALUMINUM NITRIDE TO A HIGH THERMAL CONDUCTIVITY AND RESULTANT SINTERED BODIES

Abstract: Sintered aluminum nitride bodies having a TC of at least 200 W/m.K are prepared by sintering under non-reducing conditions and controlling interrelated parameters such as binder burnout atmosphere, heating rate, sintering temperature, time at sintering temperature, cooling rate and cooling temperature. The sintered bodies may also have a TC in excess of 270 W/m.K.

Description: AN IMPROVED PROCESS FOR SINTERING ALUMINUM NITRIDE TO A HIGH THERMAL CONDUCTIVITY AND RESULTANT SINTERED BODIES TECHNICAL FIELD The present invention relates generally to a process for preparing sintered aluminum nitride (AIN) bodies and to bodies resulting from the process. The present invention relates more particularly to a process for preparing sintered AIN bodies having a thermal conductivity (TC) in excess of 270 wattstmeter*°K (Wtm # K). BACKGROUND OF THE INVENTION AIN is subject to increasing interest as a microelectronic substrate material. With a TC approaching that of beryl ia (BeO) and a thermal expansion coefficient well matched to silicon, AIN represents an attractive alternative in high power or multi-chip module applications. At room temperature, single crystal AIN has a theoretical TC of 319 Wtm*K. Polycrystalline ceramics tend to have a lower TC than single crystal AIN due to a number of factors. The factors include random orientation of AIN grains, crystalline lattice impurity levels and existence of crystalline grain boundary phases with an even lower TC. F. Miyashiro et al., in "High Thermal Conductivity Aluminum Nitride Ceramic Substrates and Packages", IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. 13, No. 2, 313-19 (June 1990), suggest that three key technologies are very important if one is to realize the highest TC by sintering. The technologies are: reducing or minimizing oxygen content of AIN powders; proper choice and quantity of additives; and sintering conditions in terms of temperature, time and atmosphere. They suggest that a reducing atmosphere yields the highest TC. H. Buhretal., in "Phase Composition, Oxygen Content, and Thermal Conductivity of AIN(Y 2 0 3 ) Ceramics", J. Am. Ceram. Soc. 74[4l, 718-723 (1991), disclose sintering cold isostatically pressed cylindrical compacts under a pressure of 0.2 MPa nitrogen. They employ heating rates of 16 to 30 K per minute (K/min). K. Watari et al., in "Sintering Chemical Reactions to Increase Thermal Conductivity of Aluminum Nitride", J. Mater. Sci. 26. 4727-32 (1991). discuss chemical reactions to increase TC by decreasing oxygen contents during AIN sintering with an Y 2 0 3 additive in a reducing nitrogen atmosphere with carbon. They use a heating rate of 15 K/min and report TC values as high as 220 Wtm«K. T. A. Guiton et al., in "Optimization of Aluminum Nitride Thermal Conductivity Via Controlled Powder Processing", Mat. Res. Soc. Symp. Proc, Vol 271 , 851-56 (1992), suggest that TC depends strongly on oxygen chemistry and sintering parameters. They disclose two sets of sintering parameters, denominated as "Cycle 1 " and "Cycle 2" in Table II (page 852). Cycle 2 includes a heating rate of 2.5°Ctmin, a sintering temperature of 1850°C, a sintering time of 3 hours, a cooling rate of c/min and a cooling temperature of 1500°C. US-A 4,847,221 discloses a process for preparing sintered AIN bodies as well as the resultant bodies The process comprises firing an admixture of AIN powder and one or more rare earth compounds in an amount of 0 01 to 15 wt-% in a reducing atmosphere at a temperature of 1550°C to 2050°C for four hours or more The resultant bodies have a TC as high as 272 W/m-K US-A 4,778,778 reports a particular sintering cycle described in a copending application The cycle includes: increasing the temperature of a compacted AIN body from room temperature to a sintering temperature at a rate of no more than 250°C per hour (°Cthr), sintering the body in an inert atmosphere at a temperature of 1600°Cto 1900°C, and cooling o the sintered body at a rate of no more than 300 c Cthr The '778 patent improves upon this cycle by introducing an amount of hydrogen gas along with the inert gas up to a temperature of 1200°C, after which pure inert gas is introduced Summary of the Invention One aspect of the present invention is a sintered polycrystalhne AIN ceramic body 5 having a TC at room temperature (25°C) of > 270 Wtm # K A second aspect of the present invention is an improved process for preparing a sintered polycrystalhne AIN body having a TCof greater than 200 Wtm # K by heating an admixture of AIN powder and at least one powdered sintering aid in the presence of nitrogen gas to a sintering temperature, holding the admixture at that temperature for a period of time sufficient to convert the admixture to a sintered body, and thereafter cooling the body to ambient temperature, characterized by a combination of heating to the sintering temperature at a rate of from > 0°Ctmιn to 6°Omιn, maintaining that temperature for a period of time sufficient to convert the admixture to a sintered body having a density of > 95 % of theoretical density, and cooling the sintered body in the presence of a vacuum or an inert gas from the sintering temperature to a temperature of 1400°C at a rate of from > 0°Ctmιn to 6°Ctmιn before cooling the sintered body further to ambient temperature (25°C) Athird aspect of the present invention centers upon the sintered body resulting from the process of the second aspect Description of Preferred Embodiments AIN powder suitable for purposes of the present invention may be of commercial or technical grade It should not contain any impurities that would have a significant adverse effect upon desired properties of a resulting sintered product Although some level of impurities is present in commercial powders, that level should be less than that which produces the aforementioned adverse effect The AIN powder typically has a bound oxygen content of < 4 wt% The oxygen content is desirably < 3 wt% and preferably < 2 wt% The AIN powder also typically has a surface area (SA), measured by conventional B.E.T. absorption methods, of from 1.5 to 10 square meters per gram (m2/g). The powder SA is desirably from 2 to 9 m2/g. AIN powder meeting these specifications is preferably prepared either by carbothermal reduction of alumina (Al 2 0 3 ) or direct nitridation of aluminum metal. AIN powders may also be prepared by other processes using aluminum alkyls or aluminum halides. Preferred carbothermal AIN powders are available from The Dow Chemical Company under the trade designation XUS 35544 and XUS 35548 or Tokuyama Soda under the trade designations Grade F and Grade H. Mixtures of these and other powders may also be used. The AIN powder may be mixed with any of the art-recognized sintering aids such as, for example, oxides or fluorides of metals selected from: yttrium; rare earth metals such as lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and dysprosium; and alkaline earth metals such as calcium, strontium and barium. A combination of sintering aids may be used in place of a single sintering aid. Yttrium compounds, particularly yttria (Y 2 0 3 ), yield satisfactory results. The sintering aid or combination of sintering aids is suitably admixed with AIN powder in an amount of from 0.05 to 10 wt%, based upon combined weight of sintering aid and AIN powder. The amount is desirably from 0.05 to 3 wt%. The sintering aid preferably has a SA similar to that of the AIN powder. Preparing an admixture of AIN powder and the sintering aid(s) may be carried out by conventional procedures such as attritor milling and wet and dry ball milling. Wet ball milling with an appropriate solvent and suitable milling media provides satisfactory results. Milling media, usually in the form of cylinders or balls, should have no significant adverse effect upon admixture components or upon sintered bodies prepared from the admixture. A liquid milling or mixing medium such as ethanol, heptane or another organic liquid may be used. After mixing, the milling medium may be removed by conventional procedures to yield an admixture suitable for conversion to ceramic greenware. Oven drying and spray drying produce satisfactory results. An organic binder may also be used during processing of the admixture into a sintered body. Suitable binders are well known in the art and typically comprise high molecular weight organic materials that are soluble in organic solvents. Illustrative binders include polyethyloxazoline, industrial waxes such as paraffin, highly viscous polyglycols, polymethyl- methacrylate and polyvinyl butyral. Polyethyloxazoline is particularly suitable. The binder is suitably added to admixture components prior to milling. Ceramic greenware may be prepared by any one of several conventional procedures such as extrusion, injection molding, die pressing, isostatic pressing, slip casting, roll compaction or forming or tape casting to produce a desired shape. Particularly satisfactory results are obtained by dry pressing a spray dried admixture or tape casting a slurry. The ceramic greenware is desirably subjected to conditions sufficient to remove the organic binder prior to sintering Binder removal, also known as binder burn out, typically occurs by heating the greenware to a temperature that ranges from 50 to 1000°C to pyrolyze, or thermally decompose, the binder The temperature varies depending upon the binder Thermal decomposition may be carried out at or near ambient pressure or in a vacuum It may be carried out in the presence of atmospheric air or in a nonoxidizing atmosphere that is desirably established with an inert gas The inert gas is suitably nitrogen, a source of nitrogen such as ammonia, or a noble gas such as argon The inert gas is preferably nitrogen As a general rule, binder burn out in the presence of an inert gas yields a higher residual carbon level than binder burn out in the presence of atmospheric air Binder burnout in the presence of nitrogen is preferred for purposes of the present invention Sintered polycrystalhne AIN bodies having a TC of > 200 W/m # K are suitably prepared under nonoxidizing conditions by combining several, interrelated sintering parameters The parameters are heating rate, sintering temperature, time at sintering temperature, cooling rate, cooling temperature, cooling environment and type and amount of sintering aid Sintering of the greenware occurs in the presence of gaseous nitrogen or a source of gaseous nitrogen and is followed by cooling in a nonreducing environment The latter may be established by using either an inert gas or a vacuum The inert gases described as suitable for binder burn out are also suitable for use in this aspect of the process One means of establishing a nonreducing environment, at least in part, includes placing the greenware into a crucible fabricated from a nonreducing material prior to sintering and cooling The non- reducing material is desirably selected from boron nitride (BN), AIN, molybdenum metal, and tungsten metal BN and AIN are preferred nonreducing materials for a graphite furnace Molybdenum metal or tungsten metal are preferred nonreducing materials for a tungsten furnace Because the parameters of heating rate, cooling rate, sintering temperature, time at sintering temperature, cooling temperature and amount and type of sintering aid are closely interrelated, several, but not all, parameter combinations lead to a TC of 200 W/rπ*K or more As an additional consideration, a given combination of parameters may produce such a TC in one AIN powder but not in another This disparity stems from variations in powder properties, particularly impurity levels, and in methods of synthesizing powders as in carbothermal synthesis versus direct nitndation As a further consideration, differences in binder bum out atmosphere lead to modifications of desirable parameter combinations Using a single AIN powder as an example, various parameter combinations for nitrogen binder burn out (BBO) and air BBO that produce a TC > 200 W/m»K or more using 3 wt% Y 2 0 3 as a sintering aid are shown in Table I The AIN powder is commercially available from The Dow Chemical Company under the trade designation XUS 35548 and has the following specification: an oxygen content of 0.8 ± 0.2 wt%; a carbon content ≤ 0.08wt%; a silicon content ≤ 100 parts per million (ppm); a calcium content ≤ 200 ppm; an iron content < 35 ppm; and a SA of 2.8 ± 0.2 m2/g. Parameter combinations for nitrogen BBO that produce a TC ≥ 270 Wtm # K using the same AIN powder and either 2 or 3 wt% Y 2 0 3 asa sintering aid are shown in Table II. The amount of Y 2 0 3 varies in direct proportion to the AIN powder's oxygen content. In other words, with an oxygen content at or near 0.6 wt%, 2 wt% Y 2 0 3 suffices whereas an oxygen content at or near 1 wt% requires as much as 3 wt% Y 2 0 3 . The actual level of sintering aid is tied, at least in part, to residual carbon levels and may be readily determined without undue experimentation. Table I BBO Sinter Heat Cool Condi¬ Sinter Atmos¬ Temp Rate tion Rate Time phere (°C) (°C/min) (°C/min) (min.) 1 N 2 1817 165 ≤1.2 ≤5.4 2 N 2 1817 165 ≤0.6 ≤5.4 3 N 2 1817 165 ≤5.4 0.1 4 N 2 1817 165 ≤5.4 ≤1.1 5 N 2 1817 ≤278 ≤5.4 0.1 6 N 2 1862 165 ≤5.4 ≤5.5 7 N 2 1908 238 ≤2.3 ≤5.5 8 N 2 1908 238 ≤1.6 ≤5.5 9 N 2 1908 ≥ 133 1.6 ≤5.5 10 N 2 1908 165 ≤5.4 ≤0.5 11 N 2 1908 165 3 ≤1.1 12 N 2 1908 165 0.6 ≤5.5 13 Air 1862 165 ≤1.3 0.1 14 Air 1862 165 ≤0.8 ≤0.5 15 Air 1862 165 ≤0.6 ≤0.5 Table II Cool¬ Condi¬ Sinter Sinter Heat Cool Rate ing tion Temp Time Rate Temp (°C) (min.) (°C/min) (°C/min) (°C) 1 1850-1875 80-180 0.6 0.1 ≤ 1650 2 1850-1875 80-180 0.6 ≤ 0.2 ≤ 1275 3 1850-1875 80-180 0.8 0.1 ≤ 1275 4 1850-1875 80-180 ≤ 1.4 0.1 1160 5 1850-1875 80-180 ≤ 1.7 0.1 1160 6 1850-1875 80-180 0.6 ≤ 0.6 1160 7 1850-1875 80-180 0.6 ≤ 0.4 1160 8 1850-1875 80-180 ≤ 1 0.4 1 160 9 1863 165 ≤ 3 0.1 ≤ 1400 The combinations shown in Tables I and II are particularly suitable when the powder is XUS 35548 (The Dow Chemical Company). Some modification of the combinations may be needed depending upon factors such as oxygen content of the AIN powder, residual carbon content of the AIN greenware and amount and type of sintering aid present in the greenware. If, for example, the powder has an oxygen content at or near the upper limit of the specification and the amount of Y 2 0 3 is 3 wt% in Table I or 2 wt% in Table II, modification may be necessary only for condition 9 of Table II. As another example, modification of conditions 1 , 7, 9, 1 1 and 1 of Table I may also be necessary when such a powder is used in combination with a lesser amount, such as 2 wt%, of Y 2 0 3 . The modification, particularly with respect to Table I, may be as simple as lowering the upper limit of the cooling rate by < rctmin. If the same powder has an oxygen content at or near the lower limit of the specification, the lesser amount of Y 2 0 3 may be used with little or no modification of the combinations shown in Tables I and II. Further modifications may be made without undue experimentation. The combinations shown in Tables I and II generally suffice to yield a sintered body having a density of > 95% of theoretical density. The density is desirably ≥ 97.5%, preferably > 99% and more preferably > 99.5% of theoretical density. Sintered AIN bodies prepared under nonreducing conditions in accordance with the process of the present invention have a TC of ≥ 200 W/m # K. The TC is desirably ≥ 240 Wtm » K, preferably 270 Wtm*K, more preferably 274 Wtm»K. The TC is also desirably < 319 Wtm»K. A TC of < 285 Wtm'K is readily attainable. Sintered AIN bodies prepared as described herein also display color/translucency combinations that range from light cream and translucent to dark gray or even black and opaque. Surface appearance, also known as mottling (marked with spots or blotches of different color or shades of color as if stained), varies from a high degree of mottling to an absence of visually detectable mottling. Skilled artisans can attain a desired combination of color, degree of mottling and TC without undue experimentation. The following example is solely for purposes of illustration and is not to be construed, by implication or otherwise, as limiting the scope of the present invention. Example Ceramic greenware was prepared from ad mixtures of 2000 g quantities of various Al N powders, either 2 or 3 wt% (41 g or 62 g) Y 2 0 3 (Molycorp, 99.99% purity) and 6.7 wt% (134.2 g) of a binder composition. The binder composition was a 33/67 weight ratio blend of polyethyloxazoline and polyethylene glycol 3350 (The Dow Chemical Company). The binder was dissolved in 3000 g of ethanol after which the AIN and Y 2 0 3 powders were added. The admixtures were ball milled for five hours. The solvent was removed by spray drying. Table III. Powder Data Powder O Ca Fe S.A. ID (wt C Si (wt %) (ppm) (ppm) (ppm) (m2/g) %) A 0.63 0.03 77 158 17 2.75 B 1.04 0.05 74 107 17 2.86 C 1.19 0.05 94 189 28 3.33 D 1.25 0.04 <20 84 < 10 3.67 E 0.91 0.03 36 220 < 10 2.72 The dried powders were dry pressed into greenware using a 7/8 inch (2.2 cm) round die under uniaxial pressure at 6.9 megapascals (MPa). The binder composition was removed from the greenware in the presence of either air (air BBO) or nitrogen (N 2 BBO). Binder removal employed a heating rate of 2 c Ctmin up to 550°C, a one hour hold at that temperature and a cooling rate of 2°C/min down to room temperature (25°C). The AIN powders and their chemical composition are shown in Table III. AIN powders A and B were different lots of powder commercially available from The Dow Chemical Company under the trade designation XUS 35548. AIN powder C was a powder commercially available from The Dow Chemical Company under the trade designation XUS 35544. AIN powders D and E were commercially available from Tokuyama Soda Co., Ltd. as, respectively, grades F and H. A BN box measuring 7.5 inch by 4.5 inch by 3.5 inch (19.0 cm by 1 1.4 cm by 8.9 cm) was used as a container to establish a non-reducing environment. The greenware resulting from binder removal was placed in BN setters, one for air BBO and one for N 2 BBO. A one cubic foot (0.028 cubic meter) graphite furnace (Thermal Technology Model 121212G) was used for sintering. The conditions that were used for sintering are shown in Table IV. Table V shows the results of sintering N 2 BBO greenware. Table VI shows the results of sintering air BBO greenware. Column headings in Tables V and VI, such as A- 3 or B-2 refer to the AIN powder type (Table III) before the hyphen and the amount of sintering aid, in wt%, after the hyphen. Two TC values are shown for most sintering runs in Tables V and VI. This reflects measurements made on two different pieces of greenware made from a single admixture and sintered at the same time. Table IV. Sintering Package Design Sinter¬ Heating Soak Soak Cooling Cooling ing Rate Temper¬ Time Rate Temper¬ Total Run (°C/min) ature (minutes) (°C/min) ature Time (°C) (°C) (hours) 1 1 1825 300 0.5 1600 26 2 3 1862 165 2.8 1400 12.8 3 5 1825 30 5 1200 8.3 4 5 1900 30 5 1600 8.6 5 3 1862 165 2.8 1400 12.8 6 1 1900 300 5 1600 20.9 7 3 1862 329 2.8 1400 16.7 8 5 1900 30 0.5 1200 30.9 9 5 1825 30 0.5 1600 13.7 10 1 1900 30 5 1200 17.8 11 3 1862 1 2.8 1400 11.3 12 3 1862 165 2.8 1157 14.2 13 3 1862 165 0.1 1400 88 14 5 1825 300 5 1600 11.4 15 5 1825 300 0.5 1200 31.6 16 5 1900 300 0.5 1600 21.2 17 3 1862 165 2.8 1643 11.3 Table IV. Sintering Package Design Sinter¬ Heating Soak oak Cooling Cooling ing Temper¬ Total Rate Temper¬ S Time Rate Time Run (°C/min) ature (minutes) (°C/min) ature (hours) (°C) (°C) 18 3 18623.0 165 2.8 1400 12.8 19 5 1900 300 5 1200 13.3 20 1 1825 30 0.5 1200 35 21 3 1817 165 2.8 1400 13.4 22 0.6 1862 165 2.8 1400 26.8 23 1 1825 300 5 1200 20.7 24 1 1900 30 0.5 1600 25.6 25 3 1908 165 2.8 1400 14.5 26 1 1825 30 5 1600 14.8 27 3 1862 165 5.5 1400 11.4 28 3 1862 165 2.8 1400 12.8 29 5.4 1862 165 2.8 1400 11.2 30 1 1900 300 0.5 1200 43.1 Table V. Measured Thermal Conductivities (W/m.K) for Sintered Parts Prepared From Nitrogen Debindered Greenware Sintering Run A-2 B-2 B-3 C-3 D-3 E-3 1 223-231 211-225 200-208 206-220 206 200-205 2 217-218 208-213 208-212 206-211 206 197-198 3 189-193 189-195 192-196 183-184 189 180-182 4 181-181 160-164 180-181 173-174 180 188-189 5 218-223 215-219 213-217 217-218 224 207-214 6 228-238 224-233 221-228 226-231 219 207-214 7 202-208 182-189 203-205 189-194 200 208-208 8 201-202 184-185 204-209 204-207 199 205-208 9 208-210 214-215 208-210 210-213 214 204-206 10 213-225 209-213 205-206 209-212 214 198-198 11 182-186 165-167 181-181 177-181 182 192-195 12 233-233 216-223 218-219 214-222 230 216-219 13 269-274 258-267 274-277 260-270 285 256-263 14 214-221 206-206 208-213 205-205 210 202-199 15 181-200 187-206 212-212 186-194 207 208-208 16 204-208 206-207 201-204 197-200 211 198-199 17 213-217 215-215 209-213 206-209 216 202-204 18 217-220 221-223 212-214 207-209 226 205-205 19 228-231 232-236 221-228 216-220 253 216-217 20 242-245 244-244 245-247 236-236 240 229-229 21 189-195 177-178 187-193 177-182 189 193-198 22 222-231 225-230 222-224 220-222 217 214-215 23 223-223 217-227 216-219 213-217 223 208-208 24 237-238 225-228 221-225 223-224 225 214-222 25 193-196 180-186 199-203 192-193 196 196-202 26 197-203 197-199 198-200 191-193 190 184-185 27 208-210 202-203 208-215 205-206 205 198-199 Table V. Measured Thermal Conductivities (W/m.K) for Sintered Parts Prepared From Nitrogen Debindered Greenware Sintering Run A-2 B-2 B-3 C-3 D-3 E-3 28 217-220 202-203 205-207 206-207 203 21-2-203 29 218-220 200-204 211-217 206-210 203 202-205 30 235-235 225-233 227-228 229-235 235 218-221 Table VI. Measured Thermal conductivities (W/m.K) For Sintered Parts Prepared From Air Debindered Greenware Sintering Run A-2 B-2 B-3 C-3 D-3 E-3 1 178-179 158-166 180-185 179-187 182 182-183 2 160-163 153-155 171-172 167-175 164 169-170 3 153-153 134-135 161-164 162-164 161 158-162 4 138-141 125-125 161-161 147-148 148 160-161 5 163-166 154-155 179-181 169-170 175 173-177 6 182-187 169-177 186-189 188-189 183 182-185 7 157-160 144-145 175-179 160-163 167 173-173 8 162-164 152-155 179-182 167-170 168 181-185 9 182-183 159-166 185-194 184-189 182 178-179 10 165-166 153-153 180-183 168-169 171 175-177 11 146-147 120-123 162-163 154-159 159 164-166 12 167-167 156-160 186-188 181-182 168 180-184 13 207-211 198-200 217-217 214-214 218 206-207 14 177-182 162-165 179-182 181-184 183 178-186 15 179-185 169-170 185-189 172-177 175 188-194 16 138-141 125-134 149-152 133-140 131 135-147 17 159-161 148-151 172-177 163-164 162 162-173 18 161-164 154-156 175-176 164-168 164 171-176 19 171-180 165-170 179-181 171-179 193 180-184 20 177-189 178-184 192-193 186-192 180 192-199 21 165-168 140-139 171-174 170-178 177 172-175 22 172-173 160-162 186-187 175-178 175 179-183 23 176-176 166-172 185-190 175-177 189 182-190 24 177-179 175-182 190-191 178-186 176 185-186 25 156-157 145-148 175-179 159-161 163 172-175 26 167-167 149-150 169-175 174-176 177 170-171 27 161-162 153-154 174-182 165-196 173 174-175 Table VI. Measured Thermal conductivities (W/m.K) For Sintered Parts Prep. a d From Air Debindered Greenware Sintering Run A-2 B-2 B-3 C-3 D-3 E-3 28 164-167 156-159 181-181 174-175 177 178-181 29 165-165 153-158 175-181 170-170 170 177-180 30 188-196 181-185 200-200 187-194 195 192-192 The data in Table V show that a TC ≥ 200 W/m « K was readily attainable under most of the sintering conditions of Table IV. In fact, sintering run 13 provided TC values of 270- 285 W/m # K for several powders. Sintering runs 3, 4, 1 1 and 21 were notable exceptions in that the sintering conditions failed to yield any sintered materials with a TC > 200 W/m*K. Sintering runs 14, 25 and 26, for example, showed that some variability in TC was present even when two pieces of substantially identical greenware were subjected to the same sintering conditions. Sintering runs 2, 7, 8, 10, 15, 16 and 27 showed that variations in powder properties lead to differing results under identical sintering conditions. Based upon the number of sintering run/powder combinations that do provide a TC ≥ 200 W/m*K, combinations that do not are readily modified to at least that level by skilled artisans without undue experimentation. The data in Table VI show that a TC > 200 W/m*K was rarely attainable under the conditions shown in Table IV. The success of sintering run 13 suggests that a skilled artisan might make modifications such as an increase in amount of sintering aid or a decrease in cooling rate, heating rate or both to achieve a TC ≥ 200 W/m # K. A comparison of run 13 from Table V with run 13 from Table VI suggests that a TC of > 270 W/m # K may not have been attainable when an air BBO procedure was used with those combinations. However, with elevated levels of Y 2 0 3 or some other appropriate sintering additive, such TC's may be possible. Analysis of all of the sintered bodies, including those having a TC > 270 W/m # K, by powder X-ray diffraction and electron microscopy revealed the presence of crystalline AIN and secondary boundary phases. The boundary phases contained Y 4 AI 2 θ9, YAI0 3 or both. The boundary phase was along grain boundaries, at triple points or both. The analysis did not reveal the presence of yttrium nitride. Similar results are expected with these powders and the sintering conditions of Tables I and II. Although some changes in sintering conditions may be required, similar results are also expected with other Al N powders.

Claims: CLAIMS: 1. An improved process for preparing a sintered polycrystalline aluminum nitride body having a thermal conductivity of greater than 200 watts/meter*°K by heating an admixture of aluminum nitride powder and at least one powdered sintering aid in the presence of nitrogen gas to a sintering temperature, holding the admixture at that temperature for a period of time sufficient to convertthe admixture to a sintered body, and thereafter cooling the body to ambient temperature, characterized by a combination of heating to the sintering temperature at a rate of from greater than 0°C per minute to 6°C per minute, maintaining that temperature for a period of time sufficient to convert the admixture to a sintered body having a density of greater than 95 percent of theoretical density, and cooling the sintered body in the presence of a vacuum or an inert gas from the sintering temperature to a temperature of 1400°C at a rate of from greater than O c C/minute to 6°C/minute before cooling the sintered body further to ambient temperature. 2. A process as claimed in Claim 1 , wherein the admixture is converted to ceramic greenware prior to sintering. 3. A process as claimed in Claim 1, wherein cooling is accomplished in the presence of an inert gas that is selected from nitrogen, argon and helium. 4. A process as claimed in Claim 1 , wherein the rate of cooling is from 0. rctminute to 5.5°Ctminute. 5. A process as claimed in any of Claims 1-4, wherein the sintered body is further present in a non-reducing environment during cooling in the presence of a vacuum or an inert gas. 6. A process as claimed in Claim 5, wherein the non-reducing environment is established by placing the admixture into a crucible fabricated from a non-reducing material prior to sintering and cooling, the non-reducing material being selected from boron nitride, aluminum nitride, molybdenum metal and tungsten metal. 7. A process as claimed in Claim 1 , wherein the sintering temperature is within a range of from 1817 to 1908°C. 8. A sintered polycrystalline aluminum nitride body prepared by the process of any of Claims 1-7 and having a microstructure characterized by a crystalline aluminum nitride phase and secondary grain boundary phases and a thermal conductivity between 270 and 319 wattstmeter #0 K. 9. A sintered body as claimed in Claim 8 wherein the grain boundary phases comprise at least one yttrium-aluminate selected from Y 4 AI 2 θ9 and YAIO3. 10. A sintered body as claimed in Claim 8 or Claim 9, wherein the grain boundary phases are located along grain boundaries, at triple points or both. 1 1. A sintered body as claimed in Claim 10, wherein the thermal conductivity is 274 wattstmeter«K or greater.

Inventors: GUITON THERESA A, MILLS LYNNE K, GUITON, THERESA, A., MILLS, LYNNE, K.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1994
Main classification: C04B35
IPC codes: C04B35